Transport models and advanced numerical simulation of silicon-germanium heterojunction bipolar transistors

Applications in the emerging high-frequency markets for millimeter wave applications more and more use SiGe components for cost reasons. To support the technology effort, a reliable TCAD platform is required. The main issue in the simulation of scaled devices is related to the limitations of the physical models used to describe charge carrier transport. Inherent approximations in the HD formalism are discussed over different technology nodes, providing for the first time a complete survey of HD models capability and restrictions with scaling for simulation of SiGe HBTs. Moreover, a complete set of models for transport parameters of SiGe HBTs is reported, including low-field mobility, energy relaxation time, saturation velocity, high-field mobility and effective density of state. Implementation in a commercial device simulator is drawn and findings are compared with simulation results obtained using a standard set of models and with trustworthy results (i.e. MC and SHE simulation results and experimental data), validating proposed models and clarifying their reliability and accuracy over different technologies. Finally, electrical breakdown phenomena in SiGe HBTs are analyzed: a novel complete model for multiplication factor is reported and validated by experimental results; new M model provides an exhaustive accuracy over a wide range of collector voltages

Towards a Universal Hot Carrier Degradation Model for SiGe HBTs Subjected to Electrical Stress

The objective of this work is to develop a generalizable understanding of the degradation mechanisms present in complementary Silicon-Germanium (SiGe) heterojunction bipolar transistors (HBTs) that can be used to not only predict the reliable lifetime of these devices but also overcome some of these aging limitations using clever device engineering. This broad motivation for understanding and improving SiGe HBT device reliability is explored through the following specific goals: 1) develop an understanding of the dominant hot carrier degradation sources across temperature (25 K – 573 K); 2) develop a broad understanding of all potentially vulnerable regions of damage within a SiGe HBT using electrically measured data, and how these degradations can be captured in a modeling framework; and 3) design optimized SiGe HBTs that can potentially overcome some of these device-level limitations in reliability across temperature. Being able to simulate the electrical degradation of a complex circuit with SiGe HBTs swinging dynamically on the output plane using a universal physics-based aging model is invaluable for any circuit designer optimizing for high performance and reliability.Ph.D

Improving linearity utilising adaptive predistortion for power amplifiers at mm-wave frequencies

The large unlicensed 3 GHz overlapping bandwidth that is available worldwide at 60 GHz has resulted in renewed interest in 60 GHz technology. This frequency band has made it attractive for short-range gigabit wireless communication. The power amplifier (PA) directly influences the performance and quality of this entire communication chain, as it is one of the final subsystems in the transmitter. Spectral efficient modulation schemes used at 60 GHz pose challenging requirements for the linearity of the PA. To improve the linearity, several external linearisation techniques currently exist, such as feedback, feedforward, envelope elimination and restoration, linear amplification with non-linear components and predistortion. This thesis is aimed at investigating and characterising the distortion components found in PAs at mm-wave frequencies and evaluating whether an adaptive predistortion (APD) linearisation technique is suitable to reduce these distortion components. After a thorough literature study and mathematical analysis, it was found that the third-order intermodulation distortion (IMD3) components were the most severe distortion components. Predistortion was identified as the most effective linearisation technique in terms of minimising these IMD3 components and was therefore proposed in this research. It does not introduce additional complexity and can easily be integrated with the PA. Furthermore, the approach is stable and has lower power consumption when compared to the aforementioned linearisation techniques. The proposed predistortion technique was developed compositely through this research by making it a function of the PA’s output power that was measured using a power detector. A comparator was used with the detected output power and the reference voltages to control the dynamic bias circuit of the variable gain amplifier. This provided control and flexibility on when to apply the predistortion to the PA and therefore allowing the linearity of the PA to be optimised. Three-stage non-linear and linear PAs were also designed at 60 GHz and implemented to compare the performance of the APD technique and form part of the hypothesis verification process. The 130 nm silicon-germanium (SiGe) bipolar and complementary metal oxide semiconductor (BiCMOS) technology from IBM was used for the simulation of the entire APD and PA design and for the fabrication of the prototype integrated circuits (ICs). This technology has the advantage of integrating the high performance, low power intensive SiGe heterojunction bipolar transistors (HBTs) with the CMOS technology. The SiGe HBTs have a high cut-off frequency (fT > 200 GHz), which is ideal for mm-wave PA applications and the CMOS components were integrated in the control logic of the digital circuitry. The simulations and IC layout were accomplished with Cadence Virtuoso. The implemented IC occupies an area of 1.8 mm by 2.0 mm. The non-linear PA achieves a Psat of 11.97 dBm and an IP1dB of -10 dBm. With the APD technique applied, the linearity of the PA is significantly improved with an IP1dB of -6 dBm and an optimum IMD3 reduction of 10 dB. Based on the findings and results of the applied APD technique, APD reduced intermodulation distortion (especially the IMD3) and is thus suitable to improve the linearity of PAs at mm-wave frequencies. To the knowledge of this author, no APD technique has been applied for PAs at 60 GHz, therefore the contribution of this research will assist future PA designers to characterise and optimise the reduction of the IMD3 components. This will result in improved linear output power from the PA and the use of complex modulation schemes at 60 GHz.Thesis (PhD)--University of Pretoria, 2014.Electrical, Electronic and Computer EngineeringPh

New design methodologies for microwave oscillators based on negative impedance. Study and development of the solution space concept

The topic of this thesis concern the study of new design methodologies for microwave oscillators based on negative impedance and LC resonant circuits. For this class of systems, it is not ordinary possibile to predict the stationary behaviour of the circuit without use non linear analysis methodologies, but these are very e complicated and not very helpful for the design. Indeed, for microwave circuits, the design technique is that of reflection parameters "S" that describe with accuracy only linear systems and shaped them in response to small signal conditions. Therefore the only way to design a good oscillator is through experience and try and error procedures. By Developping a CAD tool that allows to represent in graphical form all possibile solutions (The Solution Space) for which an amplifier and two passive networks (feedback and loads) are able to give a negative impedance looking by other port, a new design methodology has been presented. This methodology use only S small signal parameters and allows to guarantee the start-up for a given frequency when the other port is colsed with a proper LC circuit, and moreover is able to predict the behaviour of the system in steady state. Then became easily possibile to design the system in order to maximize the output power and reduce the phase noise. The proposed methodology is then successfully used in the design of a 38GHz VCO. ------------------------------------------------------------------------------------------------------------------------ L’oggetto di questa tesi è lo studio di nuove metodologie di progetto per oscillatori a microonde costituiti da un blocco di impedenza negativa e un circuito risonante LC. Per tale classe di sistemi la difficoltà nella procedura di progettazione consiste nel non aver ancora trovato una chiave che permettesse di prevederne il comportamento a regime senza l’utilizzo di metodi di analisi non lineare, notoriamente molto complicati e poco utili per il progetto. Infatti, per i circuiti a microonde, la tecnica universalmente adottata è quella dell’utilizzo dei parametri di riflessione “S” che descrivono con veridicità solamente sistemi lineari e modellati in risposta a condizioni di piccolo segnale. Non esistono quindi metodologie per gli oscillatori che vengono progettati secondo esperienza procedure del tipo try and error. Sviluppando al calcolatore un tool che permette la rappresentazione grafica dell’insieme delle soluzioni (Spazio delle Soluzioni) per cui un sistema costituito da amplificatore e reti passive di reazione e di carico possono generare una resistenza negativa ad una porta si è messa appunto una metodologia che non solo garantisce le condizioni di innesco del sistema quando la restante porta viene chiusa da un opportuno circuito risonante, di cui è possibile estrarre le caratteristiche, ma anche di prevedere unicamente utilizzando i parametri a piccolo segnale, il comportamento del sistema a largo segnale e di progettare l’oscillatore in maniera scientifica secondo le specifiche desiderate. La metodologia proposta viene quindi applicata con successo al progetto di un VCO a 38GHz